1. Field of the Invention
This invention relates to a method for making a dispersion containing at least one pigment and a carrier such as castor oil or a vegetable wax such as palm glyceride wax, and an improved dispersion produced by the process. The viscosity of the resulting dispersion is sufficiently high to prevent separation, yet sufficiently low to ensure good flow properties, including pourability.
2. Description of the Prior Art
Pigment dispersions comprise at least one pigment intimately admixed within a carrier. In the cosmetics industry, pigment dispersions have been made using batch processes based on slow mixers such as ball mills and three-roll mixers. These processes are inherently inefficient due to low throughput, long residence time, and recirculation of the material being processed. Moreover, these processes can produce a dispersion having an undesirably broad pigment particle size distribution. For maximum color strength, a narrow distribution of small sized pigment particles is required, since the color strength will be proportional to the surface area of the pigment present.
Castor oil-based pigment dispersions are used in the manufacture of lipstick. The castor oil serves to solubilize waxes and additives, and to provide a softness and smooth texture to the lipstick. The pigment is conventionally dispersed into castor oil using a two- or three-roll mill in a batch process. However, a high pigment loading (40% or more) is required to ensure high shear through the three-roll mill. The result is a very thick paste which is not pourable or pumpable, and which is believed to contain poorly-dispersed pigment particles having a broad particle size distribution. Subsequent addition of castor oil to reduce the viscosity of the paste can lead to eventual separation of the pigment from the castor oil. Moreover, the batch nature of this dispersion method is inefficient.
Cosmetic eyeliner pencils usually contain black pigment, a vegetable wax such as hydrogenated palm glyceride wax as a carrier, an alcohol such as dodecanol, and a black pigment. A three-roll mill is typically employed to disperse the pigment into the wax. Again, the batch nature of the mixing method is inefficient.
Continuous bead milling systems are commonly used to disperse both organic and inorganic pigments into aqueous systems such as water-based printing inks. See, for example, U.S. Pat. No. 4,015,999. These bead mills use a closed grinding chamber containing an internal rotor. Steel, glass or zirconium beads or shot having a diameter of from 0.5 to 1.0 micron is mixed with the pigment, and produce high impact collisions with the pigment aggregates during mixing, thereby breaking the aggregates down into individual particles. See “Milling Equipment”, The Printing Ink Manual § 13.4 (D. E. Bisset et al. eds., 4th ed. 1988).
U.S. Pat. No. 3,996,059 discloses dispersing a pigment into an organic liquid such as a hydrocarbon and halogenated hydrocarbon by bead milling.
U.S. Pat. No. 5,037,564 discloses the ball mill grinding of a dispersion of pigments (azo pigments, phthalocyanine pigments, quinacridone, perinone and perylene pigments, etc.) in low acid value lanolin fatty acid and a non-aqueous dispersion medium, optionally together with a hydrogenated castor oil fatty acid. Imai et al., 65 J. Jap. Soc. Col. Mat. 544-50 (1992) discloses a “hybridised” dispersion of barium sulfate and Lithol Rubine B in castor oil prepared by a ball mill.
U.S. Pat. No. 4,484,952 discloses the “pearl mill” grinding of hydrogenated castor oil and carbon black to produce a dispersion having a particle size range of from 0.02 to 2 mm.
An object of the present invention is to provide an efficient method for producing pigment dispersions which possess excellent color development.
A feature of the present invention is the use of effective mixing for a time sufficient to uniformly disperse at least one pigment into a carrier such as castor oil or a vegetable wax.
Another feature of the present invention is that the pigment particles dispersed in the carrier exhibit a narrow range of particle size distribution.
An advantage of the present invention is that the nature of the process permits higher throughput and lower cost than the batch processes of the prior art.